1. Field of the Invention
The present general inventive concept relates to an audio signal processing system, and in particular, to an apparatus and a method of automatically compensating an audio volume in response to a channel change.
2. Description of the Related Art
Functions of TV receivers have become more diverse and complicated. To select broadcast channels in a conventional TV receiver, a user must confirm a screen status by manually changing broadcast channels. In more recent TV receivers, the user can automatically search sequential broadcast channels of receivable frequency bands.
However, when the user changes broadcast channels, audio levels of the broadcast channels are slightly different since sensitivity of reception of the broadcast channels vary from channel to channel. Therefore, an audio volume should be compensated for the change that results from the channel change.
Conventionally, a dynamic range control (DRC) method of controlling a dynamic range of an audio signal is used to compensate for the audio volume change that results from the channel change. A dynamic range may be understood as describing a ratio of volumes of a loudest sound and a softest sound of a particular broadcast channel.
FIG. 1 is a block diagram illustrating a conventional audio volume compensation apparatus that uses the DRC method.
Referring to FIG. 1, a level measuring unit 110 measures an average envelope value of an input signal X(n) in a predetermined range. For example, the average envelope value XRMS(n) of the input signal X(n) is calculated using a|X(n)|+(1−a)·XRMS(n−1). Here, a indicates an attack time coefficient or a decay time coefficient.
A level comparator 120 compares a level of a static level curve defining a correlation between a level of an input audio signal (i.e. the input signal X(n)) and a level of an output audio signal Y(n) according to the average envelope value XRMS (n) measured by the level measuring unit 110. The level comparator 120 then outputs a gain level value according to a result of the comparison. The static level curve is defined by experimental values using G[dB]=f(X[dB]). Referring to FIG. 2, the average envelope value XRMS(n) of the input audio signal X(n) is compared with levels in Sturn, Mturn, and Bturn zones of the static level curve. A gain level of an input signal X(n) having a low audio volume is mapped to an output signal Y(n) having a volume that is increased by 10 dB, and a gain level of an input signal X(n) having a high audio volume is mapped to an output signal Y(n) having a volume that is decreased by 10 dB.
A gain calculator 130 calculates a gain value g(n) to be applied to the input audio signal X(n) based on the gain level value mapped by the level comparator 120 and a previous gain value g(n−1).
A delay unit 140 delays the input audio signal X(n) until the gain value g(n) to be applied to the input audio signal X(n) is calculated.
A multiplier 150 generates the output audio signal Y(n) by multiplying the input audio signal X(n) by the gain value g(n) calculated by the gain calculator 130.
However, the conventional audio volume compensation apparatus that uses the DRC method generates the output audio signal Y(n) by reducing the dynamic range of the input audio signal X(n) in response to the audio volume change that results from a channel change. That is, as illustrated in FIGS. 3A and 3B, if the dynamic range of the input audio signal X(n) is 20 dB when the input audio signal X(n) has a high audio volume, the dynamic range is reduced to below 5 dB as a result of DRC processing. Sound effects of the input audio signal X(n) are distorted due to the reduced dynamic range. As illustrated in FIGS. 4A and 4B, if the input audio signal X(n) has an intermediate audio volume, the input audio signal X(n) is reproduced without a large change in the dynamic range as a result of DRC processing. As illustrated in FIGS. 5A and 5B, if the input audio signal X(n) has a low audio volume, due to fast attack and slow decay characteristics, the input audio signal X(n) having the low audio volume is increased to have an intermediate volume level, and the input audio signal X(n) having the intermediate volume level is reproduced without changing the dynamic range. When the audio signal X(n) having the low audio volume is input to the conventional audio volume compensation apparatus, the distortion of the output audio signal Y(n) is small, since a change of the dynamic range is small. However, there is a problem in that the audio volume of the output audio signal Y(n) is hardly changed.